I'm after some tips on how to quickly and efficiently change lenses from a camera bag and how to reduce the likelihood of dust getting in while standing up and not having anything to put the bag on while im out shooting.

The method I mentioned only requires 2 hands at a time. Hold one lens and the camera. With practice you won't even need to hold the camera while mounting the lens(not recommended though). You could leave one lens in the bag while holding the next.

1) Camera around neck, I let it go and leave it hanging.
2) Grab the lens, with the lens cap on.
3) Push the lens release button on the camera and twist the lens until it has reached the point of removal, but is still over the lens mount.
4) Using my pinky and ring finger on the hand that holds the new lens, I twist and remove the lens cap while dropping into my camera bag.
5) As I bring the new lens up to the camera, my left hand removes the old lens. In an instant, the lenses have changed.
6) Put the old lens cap on the removed lens, and stick it in my bag.
7) Take the picture

this minimises the time taht the camera body is open but if you camera has a ust removal system on the sensore should you not be more worried about dust getting into the open lenses?

Dust removal systems are usually next to useless in my experience (except perhaps in Olympus cameras). Have a look at this dust test photo of my 40D. As you can see the anti-dust has had no affect at all and there are many more dust spots visible at 100%. So although it is necessary to ensure that the mount end of the lens is kept clean (I agree with you completely) it is equally important to protect the sensor.

Well it's not scientific or anything but all you do is take a picture of a white/gray surface and use a small aperture. The result is here. You can clearly see the dust spots all over the sensor. After cleaning the sensor take another picture of a sheet of paper to make sure it's all gone. Remember that I have used f/32 in the above shot to identify as much of the dust as possible. This much would rarely show up in real life as those apertures are best avoided because of softness caused by diffraction, but some was showing at around f/11.